has anyone here listened, owned, or know a velleman kit form K4020 MOSFET amplifier?

Pkgum, unless any here has built this amp it will be hard to tell. Do you have access to a schematic? It is the first variable bias amp kit that I have seen, (if memory serves me right). It sounds like an interesting amp, if it ever makes it down here to WA, (Wait Awhile (West Aust) I'll have too give it a go.

You got to love an amp with these specs though; 300W output, 70% efficiency, power consumption 220W? CLASS A!

But seriously it sound like it could be a great amp, who knows?

Regards WALKER
Pk, high frequency oscillation is a common problem with some amplifier output stages, (especially those with high speed output devices-read MOSFETs). It is often referred to as parasitic oscillation and can smoke your output transistors! It is not a good thing BUT usually easily cured by limiting the output stages operating speed. A change to the output filtering or small capacitors or ferrite beads on the transistors usually does the job without effecting the amps performance.
What is concerning is that the original design was, allegedly, not unconditionally stable. These days that should not be a problem. I would like to know under what conditions it was found to oscillate. For instance was it only with electrostatic speakers that it became unstable? It would not be the first amp to have trouble driving capacitive loads.
To get the best performance out of control loops they are usually tuned to be as fast as possible, to the point of oscillation and then backed off a little. Amplifier design often follows a similar strategy.

I don't think that you should reject the amp without more information. I'm still be interested in the design, does anyone have a schematic?

Regards WALKER
Based on the specs it is a 100W amp into 8 ohms, 150 W into 4 ohm load (with the extra $45 of caps). Forget the "Music" power, the only thing that means anything is the RMS rating. I would also take the "Class A" claim with a grain of salt. The website terminology makes me think they are using variable rail voltages to keep the dissipation in the output devices down at low level settings (isn't this class G?).

Phil Ouellette
Pkgum, it's possible that the amp is class A as suggested. There are a number of tricks used in some class A designs to enable better efficiency. Some times they play with the biasing, sometimes the power supply. The quick look that I had, suggested that it was a dynamic/sliding/adaptive, (pick your preferred term, there are others) bias arrangement.

There is nothing wrong with these designs, if done well, some purists might disagree.

It just seems that the power supply might be limiting the amp, not uncommon, unfortunately. This could also mean that the output devices may be on the small size, BUT, all this is merely supposition at this stage, without a schematic.
Regards WALKER
Pkgum I'm not suggesting that the power supply is anything less than "run of the mill", (average). Most people on this forum are after some thing better.
Yes it can HANDLE 4ohms, it might not like it though.
The power supply is easy to beef up, so I would not let this worry you if the amps good.
Grey, "silk purse out of a sow's ear" possibly not, but I've been surprised before. The jury is still out for me.

Regards WALKER
Sorry about the clichés.
It seams to me that we have condemned this amp without even trying it. I don’t believe that it should be discounted just because there’s an opamp in it, (maybe I’m in the minority here). I have heard a number of very nice amps that use opamps, (Musical Fidelity springs to mind). You should remember that almost ALL recordings pass through many opamps before we get hold of them.
Does anyone have a schematic? Has anyone heard it?
I tend to take what magazine reviewers say with a little caution, they are often under advertising pressure, or have strong biases, (who doesn’t I suppose).

Jury’s still out WALKER


2008-02-17 3:53 pm
I built one...

I build this amp but replaced every component with uprated versions, including TL052's instead of TL072's, etc, polystyrenes, polycarbs and polyesters in place of electrolytics and ceramics, etc...

Even had two custom-wound transformers made by antrim in the UK... 600VA each on a 500VA core (they said their cores were good for 600VA).

Even "wired" the bottom of the PCB with extra silver-plated copper wire to improve the damping factor...

Upped the smoothing caps to 1 farad per rail... stuck with the original case but heavily modified it to take the larger torroids (lots of extra metalwork in the bottom) and put 35A bi-wired speaker terminals on the back. I had to put two rocker switches in (one for each monoblock) because otherwise, switching the whole lot on to the mains would pop a circuit breaker...

I measured the damping factor to be close to 400, P/O about 115 Watts RMS, slew rate around 20V/us and a frequency range of about 15 to 50kHz.

I still have the amp today - still sounds fantastic in my opinion. I like the fact the output clean-clips because of the tight op-amp feedback drive of the o/p FETS. My favourite trick is to put on "Owner of a lonely heart" at FULL volume through my Rogers LS7-T's. Sounds absolutely fantastic...

At the time (I'm talking 1993!!) we put the amp up against the Quad 405 (I think that's right) current-dumping amplifier and it beat it hands down. I'm not sure if that's saying anything but the person who donated the '405 for the comparison thought the 405 was a good amp...
I build this amp but replaced every component with uprated versions, including TL052's instead of TL072's, etc, polystyrenes, polycarbs and polyesters in place of electrolytics and ceramics, etc...
I know that this is a year old topic, but I try to ask.
I start to build 3 of this amplifier (mono) many years ago. For personal reason, I never ended to build the kits, until some days ago.
Now, all amplifier is ok and work, but I lost in the meantime all IC's for one of them. Fot test I used the same IC, but as I need to replace IC, I like to put something updated.
I read in this post that you updated TL072 with TL052 (and it is ok as texas declare tl05x as direct update for tl07x). I guess that you updated also tl071 with tl051.
My question is, what IC replace tl061? I think that is the TL031, but is ok? What IC do you mount on your kit?
Thank you if after 13 years you can answer...